The invention relates to a form comprising a certain surface morphology of complex lenses useful for making a light diffuser and a method for making such a form.
Optical structures that scatter or diffuse light generally function in one of two ways: (a) as a surface diffuser utilizing surface roughness to refract or scatter light in a number of directions; or (b) as a bulk diffuser having flat surfaces and embedded light-scattering elements.
A diffuser of the former kind is normally utilized with its rough surface exposed to air, affording the largest possible difference in index of refraction between the material of the diffuser and the surrounding medium and, consequently, the largest angular spread for incident light. However, some prior art light diffusers of this type suffer from a major drawback: the need for air contact. The requirement that the rough surface must be in contact with air to operate properly may result in lower efficiency. If the input and output surfaces of the diffuser are both embedded inside another material, such as an adhesive for example, the light-dispersing ability of the diffuser may be reduced to an undesirable level.
In one version of the second type of diffuser, the bulk diffuser, small particles or spheres of a second refractive index are embedded within the primary material of the diffuser. In another version of the bulk diffuser, the refractive index of the material of the diffuser varies across the diffuser body, thus causing light passing through the material to be refracted or scattered at different points. Bulk diffusers also present some practical problems. If a high angular output distribution is sought, the diffuser will be generally thicker than a surface diffuser having the same optical scattering power. If however the bulk diffuser is made thin, a desirable property for most applications, the scattering ability of the diffuser may be too low.
Despite the foregoing difficulties, there are applications where a surface diffuser may be desirable, where the bulk type of diffuser would not be appropriate. For example, the surface diffuser can be applied to an existing film or substrate thus eliminating the need to for a separate film. In the case of light management in a LCD, this increases efficiency by removing an interface (which causes reflection and lost light).
U.S. Pat. No. 6,270697 (Meyers et al.), blur films are used to transmitted infrared energy of a specific waveband using a repeating pattern of peak-and-valley features. While this does diffuse visible light, the periodic nature of the features is unacceptable for a backlight LC device because the pattern can be seen through the display device.
U.S. Pat. No. 6,266,476 (Shie et al.) discloses a microstructure on the surface of a polymer sheet for the diffusion of light. The microstructures are created by molding Fresnel lenses on the surface of a substrate to control the direction of light output from a light source so as to shape the light output into a desired distribution, pattern or envelope. While the materials disclosed in U.S. Pat. No. 6,266,476 shape and collimate light and therefore are not efficient diffusers of light particularly for liquid crystal display devices. Further, the micro structures in U.S. Pat. No. 6,266,476 are simple in nature and do not contain multiple diffusion surfaces.
U.S. Pat. No. 6,241,903 (Shie et al.) discloses a method of manufacture for a light diffusion master. The method comprises blasting with shot particles in order to form impressions. While the shot blasting does create a simple piano lens capable of diffusing light, the method in U.S. Pat. No. 6,241,903 does not utilize the chrome platting step which has been shown to create nodules that allow for the formation of complex lenses.
It is known to produce transparent polymeric film having a resin coated on one surface thereof with the resin having a surface texture. This kind of transparent polymeric film is made by a thermoplastic embossing process in which raw (uncoated) transparent polymeric film is coated with a molten resin, such as polyethylene. The transparent polymeric film with the molten resin thereon is brought into contact with a chill roller having a surface pattern. Chilled water is pumped through the roller to extract heat from the resin, causing it to solidify and adhere to the transparent polymeric film. During this process the surface texture on the chill roller""s surface is embossed into the resin coated transparent polymeric film. Thus, the surface pattern on the chill roller is critical to the surface produced in the resin on the coated transparent polymeric film.
One known prior process for preparing chill rollers involves creating a main surface pattern using a mechanical engraving process. The engraving process has many limitations including misalignment causing tool lines in the surface, high price, and lengthy processing. Accordingly, it is desirable to not use mechanical engraving to manufacture chill rollers. While the roller patterns are efficient for creating a rough surface to enhance web transport, the prior art roller surfaces are not efficient light diffusers as they typically feature one light diffusion surface.
The U.S. Pat. No. 6,285,001 (Fleming et al) relates to an exposure process using excimer laser ablation of substrates to improve the uniformity of repeating microstructures on an ablated substrate or to create three-dimensional microstructures on an ablated substrate. This method is difficult to apply to create a master chill roll to manufacture complex random three-dimensional structures and is also cost prohibitive.
In U.S. Pat. No. 6,124,974 (Burger) the substrates are made with lithographic processes. This lithography process is repeated for successive photomasks to generate a three-dimensional relief structure corresponding to the desired lenslet. This procedure to form a master to create three-dimensional features into a plastic film is time consuming and cost prohibitive. Additionally, the lithography surfaces described in U.S. Pat. No. 6,124,974 does not provide a complex curved surface for efficient light diffusion.
There remains a need for an improved light diffusion of image illumination light sources to provide improved diffuse light transmission while simultaneously diffusing specular light sources.
The invention provides a light diffuser a form comprising a surface having a morphology of complex concave or convex lenses wherein the surface asperities have an amplitude of between 1 and 40 micrometers, a spatial frequency of between 1 and 100 complex lenses per square millimeter and between 1 and 40 minor lenses per major lens. The invention also provides a back lighted imaging media, a liquid crystal display component and device.
The invention provides a form useful for manufacturing light diffusers having improved light transmission while simultaneously diffusing specular light sources and a method of making the form.